An aneurysm is a dilation of a blood vessel of at least 1.5 times above its normal diameter. The dilated vessel forms a bulge known as an aneurysmal sac that can weaken vessel walls and eventually rupture. Aneurysms are most common in the arteries at the base of the brain (i.e., the Circle of Willis) and in the largest artery in the human body, the aorta. The abdominal aorta, spanning from the diaphragm to the aortoiliac bifurcation, is the most common site for aortic aneurysms. Such abdominal aortic aneurysms (AAAs) typically occur between the renal and iliac arteries, and are presently one of the leading causes of death in the United States.
The two primary treatments for AAAs are open surgical repair and endovascular aneurysm repair (EVAR). Surgical repair typically includes opening the dilated portion of the aorta, inserting a synthetic tube, and closing the aneurysmal sac around the tube. Such AAA surgical repairs are highly invasive, and are therefore associated with significant levels of morbidity and operative mortality. In addition, surgical repair is not a viable option for many patients due to their physical conditions.
Minimally invasive endovascular aneurysm repair (EVAR) treatments that implant stent grafts across aneurysmal regions of the aorta have been developed as an alternative or improvement to open surgery. EVAR typically includes inserting a delivery catheter into the femoral artery, guiding the catheter to the site of the aneurysm via X-ray visualization, and delivering a synthetic stent graft to the AAA via the catheter. The stent graft reinforces the weakened section of the aorta to prevent rupture of the aneurysm, and directs the flow of blood through the stent graft away from the aneurismal region. Accordingly, the stent graft causes blood flow to bypass the aneurysm and allows the aneurysm to shrink over time.
Most stent and stent graft systems for cardiovascular applications (e.g., coronary, aortic, peripheral) utilize self-expanding designs that expand and contract predominantly in the radial dimension. However, other system include braided stent grafts that are delivered in a radially compressed, elongated state. Upon delivery from a delivery catheter, the stent graft will radially expand and elastically shorten into its free state. In other words, the effective length of the stent graft changes as its diameter is forced smaller or larger. For example, a stent graft having a shallower, denser helix angle will result in a longer constrained length. Once the stent graft is removed from a constraining catheter, it can elastically return to its natural, free length.
Delivering a stent graft to an artery requires accurate and precise positioning of the stent graft relative to a target location in the destination artery. For example, a misplaced stent graft can block flow to a branching artery. Some stent graft delivery systems utilize one or more markers (e.g., radiopaque markers) to establish the alignment of the stent graft relative to the artery wall. However, the location of the radiopaque markers on the stent graft can move relative to an initial marker position because of the change in the stent graft's effective length upon deployment, as described above. Accordingly, after deployment of a stent graft, the stent graft (e.g., its proximal or distal edge) may miss the target point in the artery. Therefore, there are numerous challenges associated with the accurate positioning of stent grafts that change dimensions in both the radial and longitudinal directions.